This invention relates in general to electrostatographic imaging systems, and in particular, to methods and apparatus for rejuvenating electrostatographic carrier particles.
The formation and development of images on the surface of photoconductive materials by electrostatic means is well known. The basic electrostatographic process, as taught by C. F. Carlson in U.S. Pat. No. 2,297,691, involves placing a uniform electrostatic charge on a photoconductive insulating layer, exposing the layer to a light and shadow image to dissipate the charge on the areas of the layer exposed to the light and developing the resulting electrostatic latent image by depositing on the image on a finely divided electroscopic material referred to in the art as "toner". The toner will normally be attracted to those areas of the layer which retain a charge thereby forming a toner image corresponding to the electrostatic latent image. This powder image may then be transferred to a support surface such as paper. The transferred image may subsequently be permanently affixed to the support surface as by heat. Instead of latent image formation by uniformly charging the photoconductive layer and then exposing the layer to a light and shadow image, one may form the latent image by directly charging the layer in image configuration. The powder image may be fixed to the photoconductive layer if elimination of the powder image transfer step is desired. Other suitable fixing means such as solvent or overcoating treatment may be substituted for the foregoing heat fixing step.
Many methods are known for applying the electroscopic particles to the electrostatic latent image to be developed. One development method, as disclosed by E. N. Wise in U.S. Pat. No. 2,618,552 is known as "cascade" development. In this method, a developer material comprising relatively large carrier particles having finely divided toner particles electrostatically clinging to the surface of the carrier particles is conveyed to and rolled or cascaded across the electrostatic latent image bearing surface. The composition of the toner particles is so chosen as to have a triboelectric polarity opposite that of the carrier particles. As the mixture cascades or rolls across the image bearing surface, the toner particles are electrostatically deposited and secured to the charged portion of the latent image and are not deposited on the uncharged or background portions of the image. Most of the toner particles accidentally deposited in the background are removed by the rolling carrier, due apparently to the greater electrostatic attraction between the toner and the carrier than between the toner and the discharged background. The carrier particles and unused toner particles are then recycled. This technique is extremely good for the development of line copy images. The cascade development process is the most widely used commercial electrostatographic development technique. A general purpose office copying machine incorporating this technique is described in U.S. Pat. No. 3,099,943.
Another technique for developing electrostatic latent images is the "magnetic brush" process as disclosed, for example, in U.S. Pat. No. 2,874,063. In this method, a developer material containing toner and magnetic carrier particles is carried by a magnet. The magnetic field of the magnet causes alignment of the magnetic carriers in a brushlike configuration. This "magnetic brush" is engaged with an electrostatic latent image bearing surface and the toner particles are drawn from the brush to the electrostatic latent image by electrostatic attraction.
Another technique for developing electrostatic latent images is the "touchdown" process as disclosed, for example, in U.S. Pat. Nos. 2,895,847 and 3,245,823 to Mayo. In this method, a developer material is carried to a latent image bearing surface by a support layer such as a web or sheet and is deposited thereon in conformity with said image.
While ordinarily capable of producing good quality images, conventional developing materials suffer further serious deficiencies in certain areas. The developing materials must flow freely to facilitate accurate metering and even distribution during the development and developer recycling phases of the electrostatographic process. Some developer materials, though possessing desirable properties such as proper triboelectric characteristics, are unsuitable because they tend to cake, bridge, and agglomerate during handling and storage. Adherence of carrier particles to reusable electrostatographic imaging surfaces causes the formation of undesirable scratches on the surfaces during the image transfer and surface cleaning steps. The tendency of carrier particles to adhere to imaging surfaces is aggravated when the carrier surfaces are rough and irregular. In the development methods discussed above, the toner in the developer mixture can be replaced as it is used up in developing successive latent images, and the developer can be re-used in this way over a long period of time, with many thousands of images being developed. However, with prolonged use, such two component developers gradually deteriorate. In particular, owing to the collisions which occur between the toner-bearing carrier particles and the latent image bearing electrostatographic imaging surface and other parts of the electrostatographic machinery, toner particles become impacted onto the surfaces of the carrier particles, with the result that the carrier particles become less able to triboelectrically hold toner particles. This in turn leads to a tendency for the carrier particles to deposit toner particles in the background, as well as the image, areas of the latent images which are being developed. In the case of coated carrier particles, the aforementioned collisions also tend to cause the coatings to chip off the cores of the carrier particles, again leading to a reduced ability in the carrier particles to triboelectrically hold toner particles, with a consequent tendency for unwanted deposition of toner particles in the background areas of the latent images. The collisions mentioned above are especially severe and frequent in the "cascade" development method, particularly when the developer mixture is recycled by the commonly employed bucket type conveyor of the kind described in U.S. Pat. No. 3,099,943.
Eventually, the carriers in two component developer mixtures of the kind described deteriorate to such an extent that they have to be withdrawn from service. In the past the withdrawn carrier particles have frequently simply been replaced by new ones, as it has been found to be difficult to recondition the withdrawn carrier particles to produce carrier particles meeting the required specifications in a sufficiently inexpensive manner. When withdrawn, the carrier particles have on their surfaces resinous material comprising compacted toner and/or triboelectric coating material, and it is the removal of this resinous material which has provided the main difficulties. Once the resinous material has been properly removed, without substantially damaging the carrier cores, any triboelectric coating material can be replaced in the same manner as the coating was applied when making the original carrier particles.